Drive Train

Ryan BarbaPatrick Hickey

Andrew Hill

Goals

• 90 lb weight goal– Excluding engine and CVT: ~29 lb to work with

• 25 mph top speed• Easy integration with rear suspension, frame,

and brakes

Engine

• Briggs & Stratton Model 20– Designed for Baja SAE– 10 HP

• 14 ft-lb torque– Governed at 3800 RPM– 50 lb (dry)

Transmission Types

• CVT– Optimizes ratio for

application– Lightweight– Experience from previous

years– Very adjustable

• Manual– Heavier– Ease-of-use issues

• Shifting while driving

Ratio

• Require approximately 10:1 ratio with 23” tire– Provides realistic top

speed while maintaining efficient hill-climb ability

• CVT Ratio– Low: 3.9– High: 0.9

• Top speed: 25 mph• Max torque: 585 ft-lb

Reduction

• Gearbox– Efficiency– Possibility of reuse– Robustness– More expensive– Easier to mount

• Chains and Sprockets– Easily adjustable– Cheaper– Requires tensioner and external guarding

Gearbox Design

• Commercial Options– Cheaper– Proven reliability– Not purpose-built

• Heavy

• Custom Design– Lighter weight– Customization of packaging

• Narrow– Ability to design for exact ratio– More expensive

Gearbox Design

• Double Reduction– Spur gears with 20° pressure angle– First: 75/19 = 3.95:1 ratio

• 10 diametral pitch • Face width = 0.50”

– Second: 58/23 = 2.52:1 ratio• 8 diametral pitch• Face width = 1.00”

– Overall 9.96:1 ratio• 4340 alloy steel

Gearbox Design

• Case– Billet 7075-T651 Aluminum– CNC machined

• Bearings– Timken Extra Small ball bearings– Small width and low weight

• Freewheel– White Industries Trials– Allows wheels to overrun engine

Gearbox Design

• Shafts– 4140 steel– Grooves with retaining rings for gear location– Hollow for less weight– Splines on final two shafts• CVT secondary and Freewheel are already keyed• Internal splines on output for CV joint integration

Gear Design

• Face widths calculated from AGMA standards• Material removed from the webbing to reduce

weight using FEA in Creo 2.0– All loading modeled at maximum output torque

Gear 1 (left) SF: 6

Gear 2 (right) SF: 4.1

Case

• FEA modeled with maximum bearing and brake caliper loads, CVT load of 200 lb

• Safety Factor: 6.5

Case

• Brake caliper mounted to case for easier integration

• Metal-rubber shaft seals• 0.375” flange for sealing– Permatex Black RTV Gasket

Maker• Bearing/Case interface

given 0.001” interference for pressfit (from min bearing OD)

Shafts• Splined shafts from Grob Inc.• Steps for bearing location• First three shafts have 0.75” ID bearings• Final shaft has 1.25” ID bearings• Clearance (tight slip)

– First three shafts: 0.0003” clearance from min bearing ID– Output shaft: 0.0004” clearance from min bearing ID– Gear internal splines given 0.003” clearance for slip fit

Gearbox Assembly

Gearbox Assembly

Gearbox Assembly

Drivetrain Assembly

Weight SummaryComponent Weight (lb)

Engine 50

CVT 11.8

Gearbox Case 7.6

Gears & Pinions 9.0

8 Ball Bearings 1.06

4 Shafts 3.1

Sprocket (22t) 0.4

Freewheel + Adapter (22t) 0.9

Chain (18" #410) 0.5

CVT Guard Metal (.120 aluminum) 2.75

CVT Guard Fiberglass/Plastic 0.25

Fasteners 0.75

Throttle Cable 0.15

Adjustable Throttle Stop 0.25

Total 88.51